X-ray source provided with a liquid metal target

ABSTRACT

The invention relates to an X-ray source which is provided with a liquid metal target ( 3 ) which flows through a system of ducts ( 6 ) and is conducted through a duct section ( 8 ) whose flow cross-section is reduced relative to that of the system of ducts. The X-ray source is characterized notably in that there is provided a pressure source ( 12, 13; 14; 16 ) for acting on the liquid metal target ( 3 ) in such a manner that in the operating condition of the X-ray source the pressure in the liquid metal target ( 3 ) at the area of the reduced flow cross-section equals essentially a selectable reference value (P s ) or remains essentially in a pressure range between selectable limit values (P s1 , P s2 ) of the pressure. A comparatively small thickness of a window which is transparent to an electron beam can thus be realized in conjunction with a comparatively high flow speed, that is, without having to accept the risk of cavitations. The invention also relates to an X-ray apparatus provided with an X-ray source of this kind.

[0001] The invention relates to an X-ray source which is provided with a liquid metal target which flows through a system of ducts and is conducted through a duct section whose flow cross-section is reduced relative to that of the system of ducts. The invention also relates to an X-ray apparatus provided with such an X-ray source.

[0002] An X-ray source of this kind is known from DE 198 21 939.3. At the area of the reduced flow cross-section therein there is arranged a window, for example, of diamond, which is transparent to electrons and where through high energy electrons (≈150 keV) can be directed into the liquid metal target so as to excite X-ray bremsstrahlung therein.

[0003] The aim is to construct the electron window to be as thin as possible (≈1-3 μm) so as to minimize the absorption of electrons and X-rays in the window (and hence also the heating thereof) and to achieve a high power of the X-ray source. The reduction of the cross-section results in a turbulent flow of the liquid metal target at the area of the window, said turbulent flow ensuring cooling of the window and a very effective dissipation of heat, so that the power density and the continuous loadability of the X-ray source can be further increased.

[0004] However, because the window separates the liquid metal target from a vacuum chamber, it must also have a minimum thickness which is so large that the reliability of operation, notably adequate pressure strength, is ensured in all realistic operating conditions. Optimization of the window in respect of an as small as possible thickness while providing at the same time adequate strength is particularly difficult notably because the turbulent flow involves the risk of formation of cavitations which are capable of exerting substantial forces on the window and the surrounding parts, for example, when the flow speed is unintentionally increased or the reduction of the flow cross-section becomes excessive because of the presence of foreign matter or manufacturing tolerances.

[0005] Therefore, it is an object of the invention to provide an X-ray source of the kind set forth in which the minimum thickness of the window can be further reduced without affecting the reliability of operation of the X-ray source.

[0006] It is also an object to provide an X-ray source of the kind set forth in which the cooling of the window can be further enhanced by a turbulence in the liquid metal flow.

[0007] This object is achieved by means of an X-ray source which is provided with a liquid metal target which flows through a system of ducts and is conducted through a duct section whose flow cross-section is reduced relative to that of the system of ducts, characterized in that there is provided a pressure source for acting on the liquid metal target in such a manner that in the operating condition of the X-ray source the pressure in the liquid metal target at the area of the reduced flow cross-section equals essentially a selectable reference value or remains essentially in a pressure range between selectable limit values of the pressure.

[0008] A special advantage of this solution resides in the fact that on the one hand the flow speed of the liquid metal target, and hence the cooling of the window, can thus be further increased without having to accept the risk of cavitations, necessitating an increased thickness of the window, because it can be ensured by the pressure source at least that the pressure will not drop below a selectable minimum pressure and that, moreover, a selectable maximum pressure is not exceeded either, if desired.

[0009] The dependent claims relate to advantageous further embodiments of the invention.

[0010] In the embodiment in conformity with claim 2 the liquid metal target can be subject either to a non-controlled, that is, essentially constant, additional pressure which also increases the pressure at the area of the reduced flow cross-section accordingly, thus preventing the occurrence of cavitations at that area, or pressure self-control is realized, that is, notably in the further version of this embodiment in conformity with claim 3, without sensors and separate control devices or the like being required.

[0011] The liquid metal target can be subjected to a controlled pressure notably in the embodiments disclosed in the claims 4 and 5, so that the pressure at the area of the reduced flow cross-section cannot increase excessively, not even when, for example, the flow speed decreases (external pressure control).

[0012] In this case the liquid metal target is preferably subjected in conformity with claim 6.

[0013] Further details, features and advantages of the invention will become apparent from the following description of preferred embodiments which is given with reference to the drawing. Therein:

[0014]FIG. 1 is a diagrammatic representation of a first embodiment of the invention;

[0015]FIG. 2 is a detailed representation of a part of a second embodiment, and

[0016]FIG. 3 is a diagrammatic representation of a third embodiment of the invention.

[0017]FIG. 1 shows the parts of a first embodiment of an X-ray source provided with a liquid metal target which are of relevance in the context of the present invention. An electron beam source 1 (cathode) serves to generate an electron beam 2 which is directed onto a liquid metal target 3 (anode). The X-rays thus produced emanate from the X-ray source.

[0018] The liquid metal target 3 is pumped through a system of ducts 6 by means of a pump 5 and also traverses a heat exchanger 7 for the dissipation of heat from the target.

[0019] The system of ducts 6 also feeds a duct section 8 which includes a window 9 which is transparent to electrons and X-rays and also has a flow cross-section which has been reduced relative to that of the system of ducts, so that a turbulent flow occurs in the liquid metal target at the area of the window.

[0020] The electron beam 2 is aimed at the window 9 and enters the liquid metal target 3 via said window, thus generating the X-rays 4.

[0021] The window has an as small as possible thickness (approximately 1-3 μm), so that the electron beam and the X-rays can traverse the window substantially without incurring absorption losses and hence without the associated significant heating of the window.

[0022] An as fast and as strong as possible turbulent flow of the liquid metal target 3 at the area of the window 9 also provides suitable cooling of the window, so that the power density can be further increased.

[0023] For suitable proportioning of the thickness of the window, the following pressure and speed conditions of the flow at the area of the cross-sectional restriction must be taken into account

[0024] Ignoring other types of energy (frictional losses, force of gravity etc.), in conformity with Bernoulli's law the pressure P_(c) in the liquid flowing into the cross-sectional restriction at the speed v_(c) must be lower than the pressure P₁ at which the liquid flows at the speed v₁ outside the cross-sectional restriction:

P ₁ −P _(c) =P _(Bernoulli)=ρ/2(V _(c) ² −V ₁ ²).

[0025] Therein, P₁ and P₃ denote the respective static pressure and ρ is the density of the liquid. The product of ρ/2 and v_(c) is also known as the dynamic pressure.

[0026] The ratio of the speeds v₁ and v_(c) also represents the ratio of the surface area of the constricted cross-section and the surface area of the cross-section outside the constriction. This ratio of the cross-sections can be chosen at random. The smaller the cross-sectional area of the constriction, the higher the flow speed v_(c) of the liquid metal target will be at that area. In conjunction with the comparatively high density of liquid metals, a substantial pressure reduction is thus obtained on the window 9, so that the window may be constructed so as to be comparatively thin.

[0027] The above formules, however, also demonstrate that for a correspondingly small ratio of the cross-sections the pressure in the cross-sectional constriction can in theory approach zero, or at least become so small that the vapor pressure of the liquid is reached. This may give rise to cavitations, that is, the formation of cavities in the constriction and to destruction of the window 9, of the duct section 8 or of other mechanical components in the liquid metal circuit.

[0028] In order to avoid destruction or damaging of the window 9 in the case of a very small thickness and a small cross-sectional ratio, therefore, the pressure P_(c) on the window may neither drop below a minimum (first) value P_(s1) nor exceed a maximum (second) value P_(s2).

[0029] When a pressure P₁ outside the cross-sectional constriction of approximately 80 bar is assumed in a practical embodiment, the pressure ΔP_(Bernoulli) should be<P₁ and notably the pressure P_(c) on the window at least should not drop below a first minimum first value P_(s1) of approximately from 1 to 2 bar.

[0030] In order to satisfy the above conditions, a pressure sensor 10 which is known per se is provided so as to measure the actual value of the pressure P_(c) on the window 9, said pressure sensor being connected to an electronic pressure control device 11 (servo circuit). Depending on the output signal of the sensor 10, the control device 11 controls and actuates a piston 12 which, via a cylinder 13 which is connected to the system of ducts 6, subjects the liquid metal target to an additional static pressure P_(g). The pressure P_(g) is preferably controlled in such a manner that, when the actual value of the pressure P_(c) on the window decreases to or below the minimum value P_(s1), the static pressure P_(g) is increased whereas, when the actual value of the pressure P_(c) on the window increases to or beyond the maximum value P_(s2)the static pressure P_(g) is reduced accordingly.

[0031] The two values P_(s1) and P_(s2) may also be equal. Such a value P_(s) is then selected as the reference value for the pressure, that is, preferably in such a manner that it is clearly below the maximum pressure at which the window would be damaged or destroyed but at the same time high enough to avoid all cavitations.

[0032] Instead of the piston/cylinder system 12, 13 shown, for example, a pressurized gas volume which is present in a vessel and can be electromechanically compressed and expanded (for example, by way of piezoelectric elements) can also be used as a pressure source. Because the compressibility of liquids is comparatively small, a small change of the liquid volume can already result in a large pressure variation. When a relative volume variation dV/V of a constant quantity of liquid is related to its relative pressure variation dP/P, a value of approximately 10⁻³ is obtained for (dV/V)/(dP/P).

[0033]FIG. 2 shows a second, simplified embodiment, only the duct section 8 with the cross-sectional constriction 81 and the window 9 as well as a part of the system of ducts 6 which adjoins the duct section being shown therein. In this embodiment a vessel 14 is provided as the pressure source, which vessel is provided with a liquid coupling to the system of ducts 6 via a connection duct 15. The vessel 14 is provided with a diaphragm 141 which separates the liquid from a pressurized gas volume 142. The gas volume subjects the liquid in the entire system of ducts to an essentially constant, non-controlled, additional static pressure P_(g) which also increases the pressure in the reduced flow cross-section and is chosen to be such that the pressure P_(c) on the window does not drop below the minimum value P_(s1) which involves the risk of cavitations, that is, not even in the case of an increasing flow speed of the liquid metal target.

[0034] It may again be advantageous to control the static pressure P_(g) in the gas volume 142, for example, by means of a servo circuit. This circuit is supplied with the pressure P_(c) measured on the window by means of a known pressure sensor, as well as with a selected, safe operating pressure P_(s) which acts as the reference value. The static pressure P_(g) is then controlled in such a manner that it is increased accordingly when the pressure P_(c) on the window drops below the reference value P₁ and is reduced accordingly when the pressure P_(c) on the window exceeds the reference value P_(s) (or leaves the range between the two above limit values P_(s1) and P_(s2)). The change of the static pressure P_(g) in the gas volume can then be realized, for example, again by influencing the vessel 14 by means of piezoelectric elements or in a different manner.

[0035]FIG. 3 shows a third embodiment of the invention; parts therein which correspond to FIG. 1 are denoted by the same reference numerals and hence need not be elucidated again.

[0036] Like the second embodiment shown in FIG. 2, this embodiment includes a vessel 16 with a liquid coupling, via a connection duct 17, to the duct section 8. The vessel is provided with a diaphragm 161 which separates the liquid from a gas volume 162 with an essentially constant, selectable pressure. The connection duct 17 opens into the duct section 8 in a location which is situated essentially opposite the window 9.

[0037] For as long as the pump 5 is inactive, the pressure in the gas volume 162 propagates, like in the second embodiment, as a static pressure through the entire liquid circuit, as in the second embodiment. When the pump is activated and the flow increases, the pressure in the cross-sectional constriction of the duct section 8 decreases for the above reasons, and hence also the pressure on the window 9 which is situated in this area Consequently, liquid is drawn from the vessel 16, via the connection duct 17, and enters the circuit. Because of this additional amount of liquid, a comparatively strong increase of the static pressure occurs in the system of ducts 6 as well as in the duct section 8. A very small inflow or return to the vessel 16 already suffices to keep the pressure P_(c) at the area of the cross-sectional constriction at a desired reference value P_(s) or between the two above-mentioned limit values P_(s1) and P_(s2).

[0038] Pressure self-control is thus realized, the gas pressure in the vessel 16 being chosen in dependence on the cross-sectional and pressure ratios in the system of ducts in such a manner that the reference value, or the above limit values, are adhered to. The circuit preferably does not include any further pressure equalization vessels. The more rigid the system of ducts 6 and the duct section 8, the smaller the amount of liquid which is actually exchanged between the vessel 16 and the circuit will be.

[0039] An essential advantage of this embodiment resides in the fact that on the one hand no sensors, no electronic control circuitry and no hydraulic system or the like are required for controlling an external pressure source, while on the other hand the operation of the self-control system is very fast.

[0040] The principle of the invention can thus be implemented in very different ways in dependence on the desired accuracy and the relevant application. Whereas the simplest embodiment as shown in FIG. 2 can be realized without pressure control, automatic pressure self-control takes place in the embodiment shown in FIG. 3 whereas external pressure control by means of a sensor and a corresponding control device takes place in the embodiment shown in FIG. 1.

[0041] The X-ray source in accordance with the invention can thus be used in a wide variety of different X-ray devices. 

1. An X-ray source which is provided with a liquid metal target which flows through a system of ducts and is conducted through a duct section whose flow cross-section is reduced relative to that of the system of ducts, characterized in that there is provided a pressure source (12, 13; 14; 16) for acting on the liquid metal target (3) in such a manner that in the operating condition of the X-ray source the pressure in the liquid metal target (3) at the area of the reduced flow cross-section equals essentially a selectable reference value (P_(s)) or remains essentially in a pressure range between selectable limit values (P_(s1), P_(s2)) of the pressure.
 2. An X-ray source as claimed in claim 1, characterized in that the pressure source is formed by a vessel (14; 16) with a supply of liquid as well as by a pressurized gas volume (142; 162) which are separated from one another by a diaphragm (141; 161), the supply of liquid communicating with the liquid metal target (3) via a liquid coupling through a connection duct (15; 17).
 3. An X-ray source as claimed in claim 2, characterized in that the supply of liquid communicates, via a liquid coupling through the connection duct (17), with the liquid metal target (3) flowing in the reduced flow cross-section.
 4. An X-ray source as claimed in claim 1, characterized in that there is provided a sensor (10) for measuring a pressure (P_(c)) in the liquid metal target (3) at the area of the reduced flow cross-section, the output signal of said sensor being suitable to control the pressure source (12, 13; 14).
 5. An X-ray source as claimed in claim 1, characterized in that the pressure source is formed by a piston/cylinder system (12, 13) which acts on the liquid metal target and is provided with a pressure control device (11) which controls said system.
 6. An X-ray source as claimed in claim 1, characterized in that the pressure source (12, 13; 14) acts on the liquid metal target (3) which flows outside the reduced flow cross-section.
 7. An X-ray apparatus which includes an X-ray source as claimed in one of the preceding claims. 